Long objects with certain cross-sectional shapes, such as square pipes and L-shaped materials of aluminum or aluminum alloy, are generally formed by extrusion. The quality of the extruded products depends on the quality of a die and particularly on the dimensions of an opening or gap in the die.
The dimensions of the opening in the die have conventionally been measured indirectly by measuring the cross-sectional dimensions of an extruded products.
However, indirect measurement of the dimensions of the die opening is not efficient and thus there is a demand for directly measuring the dimensions of the die opening.
Regardless whether the measurement is made directly of the die opening or alternatively of the extruded product, it is difficult to attain a high precision within a tolerance on the order of one-hundredth of a millimeter where the maximum cross-sectional dimension of the die opening or the extruded product exceeds about 600 mm.
Another known means of measuring the dimensions of an object is a CCD camera. If, for example, an image of an object 10 mm long is taken by a CCD camera and processed at the resolution of 256 picture elements or pixels, then the measuring precision of 10/256=0.039 mm per pixel is possible. When the same image is processed with 512 pixels, the accuracy improves to 0.019 mm per pixel. In this way, the use of the CCD camera and appropriate lens system assures high precision with which the dimensions of small objects can be measured. This, however, gives rise to a problem that if this high level of accuracy is maintained, objects longer than 10 mm cannot be measured accurately. For example, in measuring the dimensions of the cross section of a square pipe 300 mm on each side, if the entire image of the object being measured is displayed on one field of view of camera, the above level of measuring precision cannot be obtained.